1. Field of the Invention
The invention relates to a hybrid vehicle and more particularly to a hybrid city omnibus comprising an IC drive engine and an electric drive motor whose power is of the same order of magnitude as that of the IC engine and preferably is generally equal thereto, and a transmission for selectively and alternatively connecting the motor and the engine with a vehicle drive axle.
2. Description of Prior Art
Such a vehicle as previously proposed for example in the German unexamined specification No. 2,153,961, comprises a summating transmission connected with the IC engine and the electric motor while a third shaft of the transmission is connected with the drive axle of the hybrid vehicle. When operating with the engine, the output shaft of the motor will be decoupled and vice versa so that then either the motor or the engine solely drives the hybrid vehicle.
Such a hybrid vehicle has a series of advantages: if such a hybrid drive is used in a city omnibus for example, which is to travel both in lightly populated areas in the city outskirts and also in heavily populated areas, such an omnibus may be electrically driven in the latter using the motor to avoid pollution of the air, whereas in the outskirts the diesel engine will be used as the prime mover.
In this case the diesel engine may be used in addition in order to charge up batteries for the motor. A particular advantage of such a hybrid vehicle is to be seen in the fact that the motor may also be used as a generator for braking the vehicle and thereby charging the batteries, supplying current to the electrical supply system or operating an electric brake and thus contributing to recovering a part of the braking power.
It is especially in the case of a city omnibus that frequent stopping is necessary when it arrives at stops or when negotiating city traffic jams. The considerable amount of braking power then produced is partly recovered, while at the same time the friction brakes of the omnibus will be less worn. It will also be seen that such a hybrid vehicle is particularly appropriate in hilly cities, as for example on starting from rest when the motor may aid the engine; in the case of a city omnibus, its diesel engine may operate with a comparatively light load, so that on starting, less smokey exhaust gas is evolved than would otherwise be the case with a diesel engine.
Such a hybrid vehicle is, however, substantially more elaborate in its construction than a conventional vehicle, which is only driven by an IC engine, such more complex structure being not only due to the motor, its batteries and a device for switching over the drive, but also for two further reasons: it has been seen in the past that the axle drives of such hybrid vehicles have a much higher wear rate than is the case with conventional vehicles, even if the power supplied to the driving axle was not greater than that in the case of a conventional IC engine. Investigations have led to the discovery of the reason for this: the electric motor has to have a power which is of the same order of size as that of the IC engine in order to produce the required degree of maneuverability of the vehicle when running with its electric drive. This circumstance makes it imperative to provide a motor whose armature is comparatively heavy and has a comparatively large diameter. If one furthermore takes into account the fact that the motor is run at a comparatively high speed of rotation, it will become apparent that the moment of rotational inertia of the motor is substantially greater than that of an IC engine with the same driving power. The driving chain (i.e. means connecting the prime mover with the wheels) of a vehicle forms a resilient system whose loading will increase with the moment of rotational inertia of that prime mover. Accordingly, the driving chain of the vehicle will be very much more heavily loaded if there is an electric motor at one end than if the drive is an IC engine. A further factor is that during braking very much greater braking powers sometimes have to be transmitted by the driving chain to the electric motor running as a generator.
Commercial vehicles with such a hybrid drive are necessarily higher in price, without producing a higher driving power, than regular commercial vehicles operated solely with an IC engine; the only consolation is that the fuel consumption is somewhat lower due to the utilization of the braking power. Accordingly, it is not possible to make any savings in design which would reduce the working life of the vehicle, since such a vehicle would then be worthless. It is for this reason that the designer is forced to make the axle drive of such a vehicle and the driving chain from the drive axle to the electric motor so sturdy that it has a sufficient service life. However, such a design necessarily leads to bulky transmissions so that in the case of commercial vehicles a part of the payload space has to be devoted to parts of the drive. In the specific case of a city omnibus with a rear engine and a drive rear axle this will mean that the floor of the vehicle will have to be at a higher level over the rear axle so that access to the seats at the very back of the vehicle will be impaired. This part of the vehicle floor will not be able to be used by standing passengers either.